The present invention relates to electrostatic plotters, and in particular to controllers for color electrostatic plotters.
In an electrostatic plotter, a paper passes by a head which has a large number of styli. Each stylus corresponds to a pixel to be placed on the paper as it passes by. Thousands of styli can be on a single head. Different combinations of styli are activated at each location so that an electrostatic charge is placed on the paper at each position activated. The paper then passes over toner which will adhere to pixels at which an electrostatic charge has been placed, thereby forming a picture. Color pictures can be produced by using several different toners corresponding to the different colors and making several passes across a single head or a single pass over several heads. A controller for the plotter must tell the plotter for each particular stylus whether it is to be activated or not.
The data which is presented to the plotter controller from a host computer or other input is typically in the form of graphic data descriptions. Such graphic data descriptions describe shapes on the picture to be printed, such as lines, arcs, polygons, ellipses, etc. Unusual shapes can be specified by specifying the edge of the area and what color the areas is to be filled with, for instance. The plotter controller must take this graphic data and transform it into raster data. The raster data is simply an on/off bit for each of the pixels in the picture.
Another device which uses raster data is a cathode ray tube (CRT) which is used in television sets, computer terminals, etc. The CRT has a large array of pixels formed by dots of phosphor on the screen. The raster data instructs the electron gun which is firing at the phosphor whether it is to be on or off at each particular pixel location. If the gun is on, it will cause the phosphor to emit light when it is impacted with electrons, otherwise, if the phosphor is not impacted, no light would be emitted. Color can be produced by arranging several phosphor dots at each pixel location with different types of phosphor corresponding to different colors. Alternately, the energy of the electron beam could be varied to produce color or gray scale variations. Thus, in addition to the on/off bit for each pixel, one or more bits must specify the color to be produced at each pixel and/or the intensity of each pixel.
The CRT, like the plotter, must scan the pixels line by line to produce the image. Thus, the controller must take graphical descriptions of data and convert it into raster data and feed it into the CRT or plotter in increasing values of x. The raster data must be repeatedly fed to a CRT to refresh a screen, otherwise the image would disappear. Accordingly, speed is very important for a CRT controller, and special purpose dedicated controllers on a single integrated circuit chip have been designed to provide this function. Advanced CRT controllers (ACRTC) have been integrated on a single chip to perform the functions of converting graphic data descriptions into raster data as well as controlling the display and refreshing of the CRT.
An example of a control system for a plotter is shown in FIG. 1. A host computer 10 produces graphic data descriptions which it supplies to a controller 12. Controller 12 then converts the graphic data into raster data and stores it on a disk 14. Data from disk 14 is supplied by controller 12 to a frame buffer 16 which has a fast access memory. Plotter 18 is then supplied data from buffer 16 at a fast speed as it is needed.
Because the market for plotters is not as large as the market for CRTs, fast integrated controllers for plotters corresponding to the ACRTC chip for CRTs are not presently commercially available. However, it is desirable to increase the speed of electrostatic plotters, especially where several colors are used, thus requiring additional time for each of the colors to be printed.